Implantable sensors for monitoring biological agents in the human body have been under development for many years. One example is the glucose sensor that promises to provide diabetics with improved monitoring of blood sugar levels to thereby tailor insulin treatment to alleviate the symptoms and long-term damaging effects of diabetes mellitus. In one form, an implantable electrochemical glucose sensor utilizes an enzyme such as glucose oxidase (GO) to catalytically convert glucose to gluconic acid with the simultaneous consumption of oxygen, which is detected with amperometric (current measuring) electrodes. Such electrodes are commonly fabricated from the platinum-family noble metals, because of such metals' catalytic properties and resistance to corrosion. See, for example, U.S. Pat. No. 4,890,620 granted Jan. 2, 1990 to David A. Gough, the entire disclosure of which is hereby incorporated by reference.
There are many technical challenges in designing a commercially viable implantable sensor that will meet medical device regulatory and performance requirements. First and foremost it must be safe, accurate and reliable. An implantable sensor should also be compact and be entirely contained within the body, i.e. it should not require any wires or other structures to extend through the skin, which would be unsightly, uncomfortable and a continuing source of infection. Biocompatible materials must be used where portions of the sensor come into physical contact with the body. Fabrication techniques developed in the micro-electronics industry along with specialized electrode energization and signal processing techniques offer the potential to solve many of these problems, however, failures and inaccuracies associated with the electrodes and associated structures have been problematic. In particular, there have been problems in designing and mounting the electrodes, and the electrically conductive structures to which they are connected, in a manner that will allow a hermetic seal that prevents signal degradation, shorts and other failures.
U.S. Pat. No. 6,516,808 granted to Joseph H. Schulman discloses a hermetic feedthrough for an implantable substrate sensor that comprises electronic circuitry formed within, or on, a substrate. A protective coating covers the substrate, forming a hermetically sealed package having the circuitry under the coating. The circuitry has electrically conductive pads for communicating and/or providing power to the circuitry. Electrical pathways provide hermetic electrical connection to the conductive pads for external connection to the sealed circuitry. In one embodiment, the pathway is a via that is made from a biocompatible material that is made hermetic by either increasing its thickness or by ion beam deposition. Alternatively, the pathways are formed from metal traces, surrounded by a biocompatible insulation material, essentially parallel to the surface of the substrate that are connected to the conductive pads by first vias and have second ends externally accessible to the sealed package to provide external electrical connection to the hermetically sealed circuitry within. The aforementioned structures of U.S. Pat. No. 6,516,808 of Schulman are complex and expensive to fabricate.